Precast Solutions - January/February 2008 - (Page 7)

improving the performance of concrete but had several intrinsic limitations. For example, a high volume of fibers makes concrete ball up during mixing. Even when this drawback can be overcome, the random orientation of fibers limits their effectiveness. Hauser replaced fibers with multiple layers of fine wire mats in a threedimensional shape and discovered that his micro-mat reinforcement created a stronger concrete with more predictable performance. MRC also reduced the thickness of concrete cover required over steel reinforcement. In conventionally reinforced concrete, reinforcing bars must be covered by an inch (25 mm) of concrete to protect the steel against corrosion. However, just a few millimeters of coverage is required to protect the reinforcement in MRC. This is because the mortar in MRC is so dense that it is nearly impermeable to corrosive liquids and gases. Moreover, the distributed reinforcement of micro-mat reinforcing eliminates drying-shrinkage cracking and, when used within allowable loads, flexural cracking. Where additional corrosion resistance is required, stainless steel mesh can be specified for the layers closest to the surface of the concrete. The material performs exceptionally well in freeze-thaw resistance and chloride ion penetration tests. Dynamics, Ernst Mach Institute in Germany conducted explosion tests on precast panels made with conventionally reinforced concrete and with MRC. The conventionally reinforced concrete demonstrated complete structural failure, retaining no load-bearing capacity, and produced numerous high-velocity fragment projectiles. In stark contrast, the MRC panel showed only cosmetic deformation, retained 70 percent of its In micro-reinforced concrete, mats are infiltrated with a flowable, self-consolidating cementitious slurry that cures to a high-strength mortar. BLAST AND BALLISTIC RESISTANCE Many government and private sector buildings are now being designed or upgraded to provide protection against explosions due to attack or accidents. MRC has farreaching potential for blast- and ballistic-resistant applications as well as in seismic, impact and other extreme loading conditions. Buildings no longer have to be imposing and fortress-like to provide protection. When subjected to an explosion, conventional concrete shatters, weakening the structure and producing high-velocity, shrapnel-like fragments. The distributed reinforcing within MRC minimizes damage by constraining fragments that would otherwise become projectiles. Due to its ductility, MRC can also withstand tremendous shock waves without failing and, in some instances, can even return to its original shape. MRC offers superior retention of load-bearing capacity following a blast. The Fraunhofer Institute for High Speed JAN/FEB 2008 | PRECAST SOLUTIONS 7

Table of Contents Feed for the Digital Edition of Precast Solutions - January/February 2008

Precast Solutions - January/February 2008 Contents Precast Concrete Short-Span Bridges Precast Micro-Reinforced Concrete A Winning Edge The Creative Edge TMI: Too Much Infiltration! Architectural Details Cast in Stone

Precast Solutions - January/February 2008

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